Deductions for openings in masonry

The following types of deductions are commonly found in masonry constructions.

Rectangular Openings

For rectangular openings full deduction will be as follow :-

Deduction = Length x Height x Thickness of wall

Segmental Arch Openings

With the existence of segmental arch opening over the rectangular opening, deduction in masonry is provided for both the rectangular opening & the segmental arch opening.

Area of rectangular part = L x H
Area of segmental part = 2/3L x R x R3 /2L

So, the total deduction is as follow :-

[L x H + 2/3L x R] x thickness of wall

Semi-circular arch opening :

Semi-circular arch opening :
Area of semi-circular portion = πR2/2

But the approximate area of the semi circular section is as follow :-

= ¾ L x R
So, the total deduction
= [L x H + ¾ L x R] x thickness of wall

Masonry of arch

The quantity of masonry in arch is calculated with cubic meter as an individual thing.

The quantity of arch masonry is subtracted from the total masonry.

Masonry of arch 
= Centre length of arch x width of the arch x thickness of arch
So, the deduction for arch in masonry
= Lm x b x t

Lintel over opening

An RCC lintel is constructed over the door or window opening.

The quantity of RCC of lintel is subtracted from the masonry work.

A bearing of 10 to 15 cm from the edge of opening is arranged to the lintel on either end of the opening. So,

Length of lintel (L) = Span of opening + 2 x bearing deduction for lintel = L x Width of wall x thickness of lintel.

Area of semi-circular portion = πR2/2

But the approximate area of the semi circular section is as follow :-

= ¾ L x R So, the total deduction 
= [L x H + ¾ L x R] x thickness of wall

Masonry of arch

The quantity of masonry in arch is calculated with cubic meter as an individual thing.

The quantity of arch masonry is subtracted from the total masonry.

Masonry of arch

= Centre length of arch x width of the arch x thickness of arch
So, the deduction for arch in masonry
= Lm x b x t

Lintel over opening

An RCC lintel is constructed over the door or window opening.

The quantity of RCC of lintel is subtracted from the masonry work.

A bearing of 10 to 15 cm from the edge of opening is arranged to the lintel on either end of the opening. So,

Length of lintel (L) = Span of opening + 2 x bearing deduction for lintel = L x Width of wall x thickness of lintel.

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Rajib Dey
www.constructioncost.co
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Some useful guidelines to work out the dry volume of mortar

The dry volume of mortar is measured for determining the quantity of cement and sand for the formation of mortar for our masonry work.

In recent times, cement mortar is frequently utilized for different types of masonry work.

While making estimate of the volume of mortar for various types of masonry works, only the wet volume is obtained. In order to find out the necessary volume for cement and sand, the wet volume of mortar should be changed into the dry volume.

The Dry Volume of Mortar
The prime components of mortar are cement and sand.

The mortar is formed with the inclusion of water to the dry mix of cement-sand.

After adding the water to the dry mix of cement-sand, the volume of dry mix is decreased because of the existence of air voids in sand particles.

Since the volume of mortar is raised in dry conditions, it is required to multiply the wet volume with 1.30 to obtain the dry volume of mortar.

How to work out the volume of cement and sand in mortar.

Suppose, 10 cubic feet wet mortar is required and the proportion of cement and sand in the mortar is 1:4.

Given below, the detail process to work out the dry volume mortar and then sand and cement quantity for this 10 cubic feet mortar.

How to work out the dry volume of mortar
It is known that the dry volume of mortar is,
= Wet volume of mortar x 1.30
=10 x 1.30
=13.00 cubic feet.

The process for measuring cement and sand volume in mortar

The proportion of cement and sand in mortar is 1:4. It means one unit of cement will be blended with four units of sand.

Total units of elements in mortar are 1+4=5.
Therefore, the necessary volume of sand for the projected mortar quantity is,
= (13.00 ÷ 5) × 4
= 10.40 cubic feet.

Categories of chains applied in surveying:

And necessary cement quantity is,
= (13.00 ÷5) x 1
= 2.60 cubic feet.

Conclusion: To measure the dry volume of mortar, it is required to calculate the wet volume of mortar for your work item and then multiply that wet volume by 1.30.

As soon as you obtain the required dry volume of mortar, then it is possible to compute the required cement and sand quantity for the mortar on the basis of the mix proportion.



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Published By
Rajib Dey
www.constructioncost.co
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